The branched chain Alpha-keto acid dehydrogenase (BCKDH) of mammalian mitochondria, which catalyzes the oxidative decarbosylation of the transamination products of leucine, isoleucine, and valine, is the rate-limiting step in the catabolism of the branched chain amino acids. The branched chain amino acids provide an important source of energy to the organism, yet must be maintained in adequate amounts to support protein synthesis. Accordingly, the activity of BCKDH is sensitive to both dietary manipulation and hormone levels. Phosphorylation of BCKDH on the El (decarboxylase) subunit is accompanied by inactivation, and dephosphorylation leads to restoration of activity. The goal of the project is to develop a model for regulation of the enzyme, and branched chain amino acid catabolism, based on information on the nature and properties of this phosphorylation dephosphorylation cycle. Specific questions to be addressed include the role of Ca2+ in regulation of this cycle; the role of polyamine activation of endogenous protein kinase; the significance of the presence of two distinct phosphorylation sites on BCKDH; and the possible reasons for tissue-specific differences in phosphorylation state and responsiveness of the enzyme. Experimental approaches will include studies of the isolated enzyme, and its subunits, as well as of the branched chain Alpha-keto-acid decarboxylating activity of mitochondria.